The present invention relates to a gas sensor installed in an exhaust system of an automotive internal combustion engine to detect an oxygen concentration in the exhaust gas, or an air-fuel ratio or the like.
In general, freeze proofing agent or other salt forms a corrosive environment against a metallic cover of a gas sensor installed in an automotive vehicle. An internal stress remaining in the metallic cover causes a tensile stress triggering a stress corrosion cracking.
To eliminate the stress corrosion cracking, a conventional press shaping work was performed carefully little by little so as not to leave a residual stress in the processed metallic cover. However, this conventional method increases the total number of processes and time for the press shaping work with increased costs, and is not effective to completely remove the residual stress.
According to another conventional method, a thermal treatment is applied to a processed metallic cover after being shaped into a predetermined shape. However, this conventional method may deteriorate mechanical properties of the metallic cover and will not assure a sufficient strength when this metallic cover is caulked with other member.
To solve the above-described problems, an object of the present invention is to provide a manufacturing method of a gas sensor which is capable of easily improving the stress corrosion cracking durability of a metallic cover.
In order to accomplish the above and other related objects, the present invention provides a method for manufacturing a gas sensor comprising a sensing element having a gas contact portion exposed to a measured gas, a housing holding the sensing element, a protective cover extending from a distal end of the housing and surrounding the gas contact portion of the sensing element, and a first metallic cover covering signal output terminals and leads connected to the sensing element. The manufacturing method of the present invention comprises a step of forming a semi-finished product of the first metallic cover, and a step of applying a shot blasting to a surface of the semi-finished product of the first metallic cover.
The manufacturing method of the present invention is characterized in that the shot blasting is applied to the surface of the semi-finished product of the first metallic cover.
The shot blasting is generally referred to as a surface processing technique according to which small hard balls, e.g., steel balls, steel wire chips, glass beads or the like, are continuously blown to a surface of a metallic member through a nozzle under pressurized air or by using a turbine wheel or by using other equivalent means.
According to the manufacturing method of the present invention, the first metallic cover is subjected to the shot blast processing after being configured into a predetermined shape. Hence, it becomes possible to reduce the surficial residual tensile stress while maintaining the mechanical properties of the first metallic cover obtained through the press shaping work.
More specifically, when the shot blast processing is applied to the surface of the first metallic cover, a residual compression stress remains in a surficial layer of the first metallic cover due to collision of small balls. The residual compression stress thus given to the surface of the first metallic cover completely cancels or greatly reduces the residual tensile force caused during the press shaping processes of the first metallic cover.
It is technically possible to restrict the shot blasting to only the surficial layer so that no influence of the shot blasting is given to the inner layer of the first metallic cover. Accordingly, the first metallic cover can maintain improved mechanical properties, such as tensile strength, obtained through the press shaping work.
In this manner, applying the shot blast processing to the surface of the first metallic cover makes it possible to reduce the residual tensile stress and improve the stress corrosion cracking durability. Thus, it becomes possible to optimize the press shaping processes so as to give excellent mechanical properties to the first metallic cover.
The manufacturing method of the present invention can greatly reduce the residual tensile stress remaining in the surficial layer of the first metallic cover due to application of the shot blast processing. Thus, the stress corrosion cracking durability can be surely improved.
Furthermore, according to the present invention, it is preferable that the semi-finished product of the first metallic cover is configured into a predetermined shape having a larger-diameter portion positioned near the housing and a smaller-diameter portion positioned far from the housing before applying the shot blasting to said first metallic cover.
Especially, forming the larger-diameter portion positioned near the housing and the smaller-diameter portion positioned far from the housing realizes a compact metallic cover preferably applicable to a gas sensor.
Applying the shot blast processing to the thus shaped metallic cover surely reduces the residual tensile stress and improves the stress corrosion cracking durability.
Furthermore, according to the present invention, it is preferable that the smaller-diameter portion has a hardness higher than that of the larger-diameter portion when the semi-finished product of the first metallic cover is formed.
This arrangement provides a sufficient strength to the first metallic cover when the smaller-diameter portion serves as an inner tube of a double pipe arrangement.
Furthermore, according to the present invention, it is preferable that the semi-finished product of the first metallic cover is formed through the steps of configuring a plate or tubular material into a predetermined shape having the larger-diameter portion, subjecting the shaped material to a thermal treatment, and reshaping part of the thermally treated material to form the smaller-diameter portion.
Adopting these steps makes it easy to differentiate the hardness of the smaller-diameter portion from that of the larger-diameter portion.
Furthermore, according to the present invention, it is preferable that the manufacturing method further comprises a step of overlapping a second metallic cover on an outer surface of the smaller-diameter portion of the first metallic cover, and a step of caulking overlapped portions of the second metallic cover and the smaller-diameter portion of the first metallic cover.
Adopting these steps makes it possible for the first metallic cover to maintain improved mechanical properties obtained through the press shaping work even after the shot blast processing. Thus, it becomes possible to maintain the caulked portion in an optimized condition.
Furthermore, according to the present invention, it is preferable that the manufacturing method further comprises a step of applying a shot blasting to a surface of the second metallic cover beforehand.
Adopting this step surely reduces the residual tensile stress of the second metallic cover, thereby improving the stress corrosion cracking durability of the second metallic cover.
Furthermore, according to the present invention, it is preferable that the manufacturing method further comprises a step of overlapping the larger-diameter portion of the first metallic cover on an outer surface of a third metallic cover at one end thereof after applying the shot blasting to the first metallic cover, while connecting the other end of the third metallic cover to the housing, and a step of caulking overlapped portions of the third metallic cover and the larger-diameter portion of the first metallic cover.
Adopting these steps makes it possible for the first metallic cover to maintain improved mechanical properties obtained through the press shaping work even after the shot blast processing. Thus, it becomes possible to maintain the caulked portion in an optimized condition.
Furthermore, according to the present invention, it is preferable that the manufacturing method further comprises a step of applying a shot blasting to a surface of the third metallic cover beforehand.
Adopting this step surely reduces the residual tensile stress of the third metallic cover, thereby improving the stress corrosion cracking durability of the third metallic cover.
Alternatively, according to the present invention, it is preferable that the manufacturing method further comprises a step of directly welding the larger-diameter portion of the first metallic cover to the housing, after applying the shot blasting to the first metallic cover.
This step can reduce the total number of parts required for the gas sensor and can improve the airtightness and connecting strength.
Furthermore, according to the present invention, it is preferable that the manufacturing method further comprises a step of applying an oxidation treatment to the first metallic cover after applying the shot blasting to the first metallic cover.
Furthermore, according to the present invention, it is preferable that the shot blasting is performed in such a manner a compression stress is uniformly given to at least one of inner and outer surfaces of the first metallic cover. This is effective to realize a uniform removal of the residual stress.
Furthermore, according to the present invention, it is preferable that the shot blasting is applied to both of inner and outer surfaces of the first metallic cover.
Furthermore, according to the present invention, it is preferable that the shot blasting is performed so as to satisfy a relationship that B/A is equal to or larger than 0.8, where xe2x80x9cAxe2x80x9d represents an entire surface area of the first metallic cover and xe2x80x9cBxe2x80x9d represents a total area of dents formed on the surface of the first metallic cover.